Instrumentation for inspecting the integrity of a borehole casing can use gamma ray sources and gamma detectors. In some implementations, radiation from an x-ray or gamma ray source can be reflected from the casing to determine the casing density, weld quality, voids, etc. Conventional instrumentation can include a probe body containing a gamma ray source and a gamma ray detector separated by a predetermined length. The collected data can represent the response seen at the detector when radiation is passed from the source into the borehole's wall formation. Some radiation will be lost, but some will be scattered and reflect back toward the detectors, this reflected radiation being useful in determining properties of the formation.